Consolidated Statements of Equity (USD $) In Millions, except Share data	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Noncontrolling Interest [Member]	Total
As of December 31, 2009 at Dec. 31, 2009	$ 390	$ 679	$ 848	$ (116)	$ (21)	$ 39	$ 1,819
As of December 31, 2009 (in Shares) at Dec. 31, 2009	77.5
Net income			234			16	250
Other comprehensive loss				(33)		1	(32)
Dividends on common stock			(136)				(136)
Purchase of additional 15% ownership interest in SouthStar		(51)		(1)		(6)	(58)
Distributions to noncontrolling interest						(27)	(27)
Purchase of treasury shares					(7)		(7)
Purchase of treasury shares (in Shares)	(0.2)
Issuance of treasury shares		(1)	(2)		18		15
Issuance of treasury shares (in Shares)	0.4
Stock-based compensation expense (net of tax)		8			1		9
Stock granted, share-based compensation, net of forfeitures		(7)					(7)
Stock issued, dividend reinvestment plan	1	2	(1)		7		9
Stock issued, dividend reinvestment plan (in Shares)	0.2
Stock issued, share-based compensation, net of forfeitures		1					1
Stock issued, share-based compensation, net of forfeitures (in Shares)	0.1
Balance at Dec. 31, 2010	391	631	943	(150)	(2)	23	1,836
Balance (in Shares) at Dec. 31, 2010	78.0
Net income			172			14	186
Other comprehensive loss				(67)			(67)
Dividends on common stock			(148)				(148)
Distributions to noncontrolling interest						(16)	(16)
Purchase of treasury shares					(2)		(2)
Purchase of treasury shares (in Shares)							100,000
Issuance of shares for Nicor merger	191	1,332					1,523
Issuance of shares for Nicor merger (in Shares)	38.2						38,200,000
Stock-based compensation expense (net of tax)		8					8
Stock granted, share-based compensation, net of forfeitures		(11)					(11)
Stock issued, dividend reinvestment plan	1	9					10
Stock issued, dividend reinvestment plan (in Shares)	0.3
Stock issued, share-based compensation, net of forfeitures	3	20			(3)		20
Stock issued, share-based compensation, net of forfeitures (in Shares)	0.5
Balance at Dec. 31, 2011	586	1,989	967	(217)	(7)	21	3,339
Balance (in Shares) at Dec. 31, 2011	117						117,044,803
Net income			271			15	286
Other comprehensive loss				(1)			(1)
Dividends on common stock			(203)				(203)
Distributions to noncontrolling interest						(14)	(14)
Stock-based compensation expense (net of tax)		7					7
Stock granted, share-based compensation, net of forfeitures		(10)					(10)
Stock issued, dividend reinvestment plan	1	9					10
Stock issued, dividend reinvestment plan (in Shares)	0.3
Stock issued, share-based compensation, net of forfeitures	3	19			(1)		21
Stock issued, share-based compensation, net of forfeitures (in Shares)	0.6
Balance at Dec. 31, 2012	$ 590	$ 2,014	$ 1,035	$ (218)	$ (8)	$ 22	$ 3,435
Balance (in Shares) at Dec. 31, 2012	118						117,855,075